1. Field of the Invention
The present invention relates, in general, to organic laminate chip carrier packages which mount encapsulated semiconductor chips, such as plastic ball grid array (PBGA) packages providing for the mounting of so-called flip-chips, and wherein the chips are usually overlaid with a heat spreading cap designed to balance the coefficients of thermal expansion (CTE) and the stiffness of a substrate which is exposed on sides opposite of the chips, in order to compensate for mismatches in the coefficients of thermal expansion, and resulting in contractions which cause the entire package arrangement to warp, leading to delamination between an encapsulant and cap and resulting in failure of connect joints and the ball grid arrays.
The concept of compensating for any mismatches in coefficients of thermal expansion (CTE) and resultant contraction which are encountered between the various components of a wire bond or flip chip package or module including encapsulated semiconductor chips mounted on substrates, and with heat sinks supported on the chips in the form of caps in order to reduce heat-induced warpage tending to separate the components and leading to failures of the electrical connects and ball grid arrays adversely affecting the functioning and reliability of the packages has been widely addressed in the technology and industry. Nevertheless, notwithstanding the considerable efforts expended in order to solve the problems which are encountered, the utilization of the heat-spreading caps or heat sinks has only been partially successful in maintaining the relative flatness of the components and avoiding warping, inasmuch as the heat-spreading cap only balances the thermal dissipation above the substrate directly above the chip. The foregoing difficulties are encountered due to package warping as a result of thermal stresses generated, in that the normally utilized epoxy adhesives which cement the chip and cap to the substrate may not match the coefficients of thermal expansion of the substrate. One of the possible failure mechanisms is delamination of the epoxy interface between the chip and the cap as a result of thermally induced thermal stresses. Also, the tendency of the epoxy adhesive to absorb or desorb moisture may readily cause additional expansion and contraction and result in further warpage of the entire module structure, leading to an operative failure of the arrangement.
2. Discussion of the Prior Art
At this time, various types of structures have been employed in the technology which concern themselves with the provision of means for dissipating heat which is generated during the operation of the module, and to avoid thermal expansions and contractions which would conceivably cause the module to warp, leading to operative failure and/or reduction in the service life thereof.
In particular, the semiconductor chips have been equipped with superimposed heat sinks in the form of so-called caps or covering structures which are adhesively fastened thereto; for example, such as through the interposition of an epoxy adhesives or the like, and wherein the caps extend generally above the areas defined by the surface extent of the chips. Such caps are normally constituted of a heat-conductive material, such as steel or copper. Other types of caps may incorporate multiple layers of different materials or have fins formed thereon providing enlarged surface areas for dispersing and dissipating heat, whereas other structures may have the caps extending beyond the confines of the chip and be suitably shaped to theoretically optimize the rate of heat dissipation from the module arrangements.
Alcoe et al U.S. Pat. No. 5,877,043, which is commonly assigned to the assignee of the present application, discloses an electronic package incorporating a flexible substrate, stiffener and semiconductor chip. In that instance, there are provided a plurality of strain relief structures at various locations about the package so as to prevent surface wrinkling associated with large differences in coefficients of thermal expansion (CTE) between the various package elements. Although this provides for a heat dissipation structure, there is no disclosure of a unique cap design analogous to that contemplated by the present invention.
Tokuno et al U.S. Pat. No. 5,777,847 discloses a multichip module including a metallic cover plate fastened by means of a support pillar to a substrate, and wherein the plate is constituted of a heat conductive material, preferably such as copper or other suitable metal, such as aluminum or aluminum alloy.
Marrs U.S. Pat. No. 5,485,037 discloses a heat sink in the form of a cap or cover structure arranged above semiconductor chip which is mounted on a substrate, and which includes a plurality of holes adapted to be filled with a suitable filler material facilitating the dissipation of heat in a generally uniformly dispersed manner across the surface of the chip while maintaining the essential rigidity or stiffness of the heat sink or cap.
Baska et al U.S. Pat. No. 5,745,344 discloses a heat-dissipating apparatus including spaced fins for absorbing heat generated by an electronic device.
Liberty et al U.S. Pat. No. 5,213,868 is directed to a thermally conductive interface material of a polymeric binder and one or more thermal filters so as to form a heat sink for an electronic component.
Caletka et al U.S. Pat. No. 6,104,093 discloses a thermally enhanced and mechanically balanced flip chip package and method of forming the package. However, unlike the present invention, the description of recesses in this patent resides in the context of peripheral steps, and further non-flat; i.e. varying thickness cross-section. This patent does not teach nor suggest the use of small-sized apertures to promote adhesion, but rather promotes adhesion by enrichment of surface chromium and use of silane coupling agents rather than small-sized apertures.
Other types of structures which have been employed as heat sinks and which are arranged above encapsulated semiconductor chips and fastened thereto with the intermediary of epoxy adhesives, which may include heat conductive greases, including cap configurations incorporating pluralities of holes in varying specified sizes and arrays, and also scallops formed along the edges of the caps extending over the edge portions of the chips located therebeneath so as to allow for improved degrees of heat dissipation therefrom.
Although the foregoing types of cap or cover construction forming heat sinks are adapt in dissipating heat from the semiconductor chips and ball grid arrays, so as to reduce or possibly eliminate warpage encountered between the various electronic package components; in effect, the substrate mounting the semiconductor chips and the caps or cover structures forming the heat sink mounted above the chips to reduce warpage and potential failure of the ball grid arrays and the entire package modules, pursuant to the invention further improvements in design and construction would still further enhance the degree of heat dissipation through the caps so as to assist in eliminating warpage between the components and improve upon operational reliability, while increasing the service life of the package module or arrangement.
In order to improve upon the current state-of-the-art in the design and construction of cap structures which will extensively reduce or obviate delamination thereof from an adhesive or dielectric material bonding the cap structure to a chip; in effect, improving the interfacial adhesion therebetween, the cap structure is provided with a multiplicity of small-sized perforations forming increased surface areas for the bonding and creating of mechanical interlocking means between the components. The perforations which are provided in the cap structure, the latter of which is preferably of a flat or planar configuration, and wherein the perforations extend therethrough or at least partially thereinto, may be formed through a photolithography process and chemical etching.
An important advantage derived by the provision of a multiplicity of small apertures, which may be either in the form of straight uniformly dimensioned perforations, tapered holes, reversely tapered holes, blind holes or scalloped holes or recesses facing towards the adjacent chip surface, resides in that the numerous small holes or perforations considerably increase the surface areas for bonding in the absence of significantly reducing the mechanical stiffness and strength of the cap structure.
Another advantageous aspect of the inventive perforated cap structure may be ascertained in that, although the primary function resides in preventing warpage by balancing the extents of the thermal expansion of the superimposed layers of the components, it also serves a function as a heat spreader. In effect, the cap structure possesses some thermal conductivity, and may thus be imparted the properties of a thermally conductive member aiding in the dissipation of heat from the package or module.
In particular, pursuant to the invention, there is contemplated the provision of a novel heat-dissipating cap structure mounted above an encapsulated chip of an electronic package, wherein the cap preferably extends beyond the edges or confines of the chip, and may be essentially constituted of a flat or planar plate structure. Preferably, pursuant to one inventive aspect, the cover or caps is constituted of an extensively perforated metallic material of non-woven construction imparting a somewhat mesh-like characteristic thereto which will improve resistance to delamination between the encapsulant, such as an epoxy and the cap, causing only a slight reduction in the bending stiffness of the cap while concurrently maintaining the in-plane stiffness of the cap to an essentially high degree.
Pursuant to another aspect of the invention, it is also possible to contemplate that the heat-dissipating cap may be constructed with a plurality of perforations forming adhesive-filled holes or interstices arranged along the edge regions of the chip, and with other perforations of selectively varying sizes and patterns being optionally dispersed over the other surface regions of the cap, and optionally with selectively smaller and/or larger sized perforations of other types being dispersed over the cap area extending over the chip surface structure.
According to another embodiment of the invention, it is possible to provide the cap with perforations or recesses in the form of scallops and the like facing towards the surface of the chip, and which only partially extend through the thicknesses of the cap, thereby enabling the partially-through perforations and/or scallops to be at least partially filled with epoxy adhesive or bonding material so as to assist in the mechanically locking interconnection between the cap structure and the chip. Such scallops or partially-through extending blind perforations may be arranged around the perimeter or edge regions of the chip, or other suitably selected areas over the cap surface, as required by particular physical applications in order to facilitate an enhanced adhesive connection between the various package components while concurrently maintaining the strength of the cap, and thereby prevent any delamination-causing warpage taking place between the components of the package or module.
Accordingly, it is an object of the present invention to provide a novel perforated cap structure for reducing warpage encountered between the components of a semiconductor package module caused by different coefficients of thermal expansion (CTE) among the various module components.
A more specific object resides in the provision of a novel heat-dissipating perforated, mesh-like cap structure for an encapsulated semiconductor chip package structure wherein the cap is formed of a metallic material.
Still another object of the present invention is to provide a structure for reducing the warpage of components of a coupled cap ball grid array or flip chip package so as to reduce interfacial stresses through the provision of a novel perforated cap structure providing for enhanced mechanical interlocking through an adhesive filling the perforations, and compensating for different coefficients of thermal expansion between the various components.
Still another object is to provide a method for reducing the warpage of flip chip or ball grid array semiconductor packages having coupled cap-chip structures through the formation of uniquely configured perforations in the cap which enable the enhanced mechanical interlocking between the components in that the perforation from increased surface areas by bonding through an adhesive encapsulant.